In the production of oil and gas from subterranean wells, the composition of fluids entering the wellbore from the formation zone or zones being produced may include a substantial amount of water and/or gas mixed with crude oil. For example, certain wells are configured to produce crude oil from multiple formation zones spaced along the wellbore and the water content of the fluid mixture flowing into the well from one zone may be greater than from one or more other zones which are also producing into the well. Relatively mature oil fields which are being subjected to waterflooding may produce a substantial amount of water into a production well from one zone while an oil-water mixture having a much lower water content is being produced into the same well from another zone. Accordingly, it is desirable to be able to identify the zone which is producing an unwanted quantity of water so that certain treatment operations may be carried out to reduce such water flow. Moreover, it may also be desirable to be able to measure the water content of the fluid mixture flowing from a well to the surface to eliminate expensive fluid separation equipment and associated flow measuring devices at the wellhead.
Several systems have been developed for aboveground installations for measuring the composition of a mixture of water and oil, such as is typically produced from many hydrocarbon-producing wells. U.S. Pat. No. 4,429,273, issued Jan. 31, 1984 to R. P. Mazzagatti describes an oil-water monitor characterized by an electrical co-axial transmission line having a signal source of electromagnetic radiation in the form of a continuous wave signal with a frequency preferably of 100 kilohertz to 500 megahertz. U.S. Pat. Nos. 4,774,680 and 5,263,363 to J. Agar disclose oil-water monitors which measure energy absorption properties of an oil-water mixture in the oil continuous phase or water continuous phase. U.S. Pat. Nos. 4,862,060; 4,996,490 and 5,157,339 to Scott et al. and assigned to the assignee of the present invention, describe apparatus for measuring the concentration of water in crude oil, for example, wherein a radio frequency (RF) range signal generator, preferably operating in a microwave frequency range, is provided in the form of an unbuffered oscillator which is connected to a device through which the fluid mixture flows and forms part of a coaxial microwave transmission line. The devices described in the Scott et al. patents are all adapted for use interposed in a process flow line at the surface and not for downhole operations or measurements.
U.S. Pat. No. 5,025,222 to Scott et al. describes further applications for the above-mentioned type of fluid measurement apparatus. Still further, U.S. Pat. No. 5,103,181 to Gaisford et al. describes a method and system for measuring the properties of fluid mixtures utilizing the complex dielectric properties of these mixtures flowing through a pipe which acts as a waveguide for propagating radio frequency electromagnetic energy.
None of the above-mentioned systems or devices are adapted for "downhole" measurement of fluids flowing within a well. Certain devices have been proposed for measuring the composition of fluids downhole but these devices lack the accuracy desired for many well fluid measurement applications and they do not measure more than a small portion of the total fluid flow through the well. Moreover, these prior art devices are typically interposed in a reduced diameter conduit which is connected to a seal-forming device such as an inflatable packer. Such devices restrict the flow of fluids through the wellbore, are difficult to insert and retrieve, and cannot be continuously moved through the wellbore to monitor changes in the flow conditions and fluid compositions at any selected position in the wellbore.
Accordingly, there has been a continuing and increasing need to develop a system wherein a measurement device can be placed in a well, traversed through the well without significantly impeding fluid-flow within the well and be capable of more accurately measuring the composition of the wellbore fluids at substantially any selected position in the wellbore and while measuring substantially the entire fluid flowstream in the well. In particular, there has been a need to provide a method and system for accurately measuring the so-called watercut or percentage of water in the total fluid flow stream flowing through a well so that, in particular, for wells which are producing fluids from multiple spaced-apart zones, those zones which are producing excessive quantities of water can be identified and isolated from producing into the well, if desired. It is to these ends that the present invention has been developed.